1. Field of the Invention
The present invention relates to an active matrix liquid crystal display, and more specifically to a thin film field effect transistor array used in the active matrix liquid crystal display and having a storage capacitance formed by utilizing a gate bus line.
2. Description of Related Art
At present, liquid crystal displays have been focused as a flat display panel for portable computers and wall mounting type television displays. In particular, active matrix type liquid crystal displays composed of an array of thin film field effect transistors formed on a glass plate and each used as a switching element for an associated pixel, have been expected for application to the television displays and others, since they have possibility of a full color display. Therefore, the active matrix type liquid crystal displays are being actively studied and developed by many research organizations.
In this active matrix type liquid crystal display, a high picture quality and a high definition are current important problems. For this purpose, a method of providing a storage capacitance in parallel with a pixel capacitance has been known. The storage capacitance can be realized either by forming a storage capacitance line independently of the thin film field effect transistor, or by utilizing the gate bus line of a preceding stage thin film field effect transistor. The latter is very effective, since it is not necessary to provide, within each pixel, an opposing electrode and a bus line for the storage capacitance, and since it is possible to avoid increase of manufacturing processes.
One example of the storage capacitance realized by utilizing the gate bus line has been proposed by E. Takeda et al. in 1988 INTERNATIONAL DISPLAY RESEARCH CONFERENCE, pages 155-158. In this case, the gate electrode of a preceding stage thin film field effect transistor is extended to overlap with a succeeding stage pixel.
Here, in order to obtain a high quality of displayed image by preventing flicker and/or crosstalk, it is necessary to provide the storage capacitance as large as possible. However, if the storage capacitance is made large in the above mentioned prior art storage capacitance realized by utilizing the gate bus line, an area of the electrode for the storage capacitance must be inevitably made large, with the result that an aperture ratio in the pixel matrix section decreases. This means that a bright image having high contrast cannot be obtained.